Port fuel direct injection (PFDI) engines include both port injection and direct injection of fuel and may advantageously utilize each injection mode. For example, at higher engine loads, fuel may be injected into the engine using direct fuel injection for improved engine performance (e.g., by increasing available torque and fuel economy). At lower engine loads and during engine starting, fuel may be injected into the engine using port fuel injection to provide improved fuel vaporization for enhanced mixing and to reduce engine emissions. Further, port fuel injection may provide an improvement in fuel economy over direct injection at lower engine loads. Further still, noise, vibration, and harshness (NVH) may be reduced when operating with port injection of fuel. In addition, both port injectors and direct injectors may be operated together under some conditions to leverage advantages of both types of fuel delivery or in some instances, differing fuels.
In PFDI engines, a lift pump (also termed, low pressure pump) supplies fuel from a fuel tank to both port fuel injectors and a direct injection fuel pump. The direct injection fuel pump may supply fuel at a higher pressure to direct injectors. Further, the direct injection (DI) fuel pump may be deactivated during certain periods of engine operation (e.g., during port fuel injection at low engine loads, engine idle conditions) which may affect lubrication of the DI fuel pump and increase wear, NVH, and degradation of the DI fuel pump.
One approach to reducing DI fuel pump degradation and improving lubrication may include continued direct injection of fuel into the engine at lower engine loads. In another example approach shown by Pursifull et al. in US 2014/0224209, the DI pump may be lubricated by maintaining a pressure difference between a top and a bottom of a piston in the DI pump. Herein, the DI fuel pump may be operated in a mechanical mode while direct fuel injection is reduced and/or discontinued. The pressure difference may be achieved by maintaining a compression chamber of the DI fuel pump at a default pressure wherein the default pressure is higher than an output pressure of the lift pump. The default pressure within the compression chamber may be obtained by deactivating the solenoid activated check valve enabling the solenoid activated check valve to operate in a pass-through state. Further, a pressure relief valve may be positioned upstream of the solenoid activated check valve to regulate fuel flow received from the compression chamber via the solenoid activated check valve during a compression stroke in the DI fuel pump. As such, the default pressure in the compression chamber of the DI fuel pump may be substantially equivalent to a pressure relief setting of the pressure relief valve.
The inventors herein have identified potential issues with the above approaches. For example, in the approach where direct injection is continued at lower engine loads, excessive NVH may be generated from ticks resulting from the actuation of the solenoid activated check valve in the DI fuel pump. These ticks may be audible to a vehicle operator and passengers due to a lack of engine noise to mask the DI fuel pump noise during engine operation at lower loads. Further, in the approach wherein the compression chamber in the DI fuel pump is maintained at default pressure by the pressure relief valve, fuel heating may occur due to repeated fuel flow through the pressure relief valve. Herein, the pressure relief valve provides a restriction to the fuel flow contributing to heating of the fuel. Furthermore, an increase in the temperature of the fuel may cause formation of fuel vapor, which can adversely affect pump lubrication. Further still, fuel heating may increase power consumption.
The inventors herein have recognized the above-mentioned issues and identified an approach to at least partly address the above issues. In one example approach, a system is provided comprising an accumulator positioned within a bore of a direct injection fuel pump in a coaxial manner, the accumulator positioned downstream from a solenoid activated check valve. The accumulator within the direct injection fuel pump may provide the default pressure to enable lubrication of the direct injection fuel pump during lower engine loads.
In another example approach, a method is provided comprising, when a solenoid activated check valve positioned upstream of an accumulator is de-energized and commanded to a pass-through state, regulating a pressure in a compression chamber of a direct injection fuel pump via axial motion of the accumulator, the accumulator positioned coaxially within a bore of the direct injection fuel pump.
For example, a DI fuel pump of a fuel system in a PFDI engine may include an accumulator positioned within a bore of the DI fuel pump. The accumulator may include a spring coupled to a piston. Further, the accumulator may be arranged downstream of an electronically controlled solenoid activated check valve. The DI fuel pump may be operated in one of two modes: a default pressure mode and a variable pressure mode. The solenoid activated inlet check valve may be activated, and maintained active, during the variable pressure mode. When the solenoid activated check valve is energized, it may regulate the fluid volume pumped into the direct injection fuel rail. Thus the solenoid activated check valve may be a fuel volume regulator. In other examples, the solenoid activated check valve may control pressure in the direct injection rail synchronously with a pump stroke in the DI fuel pump. When included in a closed loop pressure control system with a pressure sensor, the solenoid activated check valve may be an active element in a fuel rail pressure control system. In the default pressure mode, the solenoid activated inlet check valve may be deactivated to function in a pass-through state and the DI fuel pump may be operated with a default pressure. The default pressure mode may be activated during lower engine loads and engine idling conditions when direct injection into the chamber is reduced and/or disabled. The accumulator within the DI fuel pump bore may regulate pressure within the compression chamber, and the direct injection fuel rail, via axial motion of the piston of the accumulator. As such, the accumulator may store fuel at the default pressure through at least a portion of a compression stroke releasing the fuel into the direct injection fuel rail when fuel rail pressure decreases below the default pressure. A pressure relief valve may or may not be included in the DI fuel pump of the fuel system. By including the pressure relief valve, fuel heating after shutdown may be achieved.
In this way, a DI fuel pump may be operated during lower engine load conditions. By preserving a default pressure in the compression chamber via the accumulator, the DI fuel pump may be lubricated when fuel flow out of the direct injection fuel pump to fuel injectors is reduced and/or ceased. Specifically, an interface between a piston and a bore of the DI fuel pump may be lubricated. Since the DI fuel pump may be operated with a deactivated solenoid actuated check valve in the default pressure mode, a reduction in audible ticking noises and NVH may be provided. Further, by regulating pressure within the compression chamber via the accumulator in the default pressure mode, fuel heating due to repeated pump strokes may be diminished. By reducing a likelihood of fuel heating, vapor formation may be moderated. Furthermore, adverse effects of vapor formation on pump lubrication may be eased. Overall, durability of the DI fuel pump may be extended while simultaneously enhancing its performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.